Among various types of prior art horn antennas, a tube antenna utilizing a dielectric element is known. A typical example of this type of a horn antenna is disclosed in Patent Document No. 1 listed below (U.S. Pat. No. 8,354,970 B2). The horn antenna disclosed there includes: a metallic circular waveguide having a flared, cone shaped front end; a dielectric circular waveguide having a rear end in connection with the front end of the metallic circular waveguide; and a dielectric horn provided at the front end of the dielectric circular waveguide.
In this horn antenna, the electromagnetic wave radiation is emitted dominantly from the dielectric horn. In comparison with a typical, fully metallic horn antenna having a certain antenna diameter, this horn antenna having about half the antenna diameter may be capable of providing nearly the same level of gain, as well as nearly the same level of directionality, as that which may be provided by the fully metallic horn antenna.